It is estimated that there are more than 6.5 million people in the U.S. diagnosed as having diabetes mellitus. Of those diagnosed, more than 90% have Type II diabetes mellitus. Although β-cell dysfunction is detectable in all diabetic patients whose pancreas exhibits an inability to produce sufficient insulin to maintain glucose levels in the normal range, the rapid increase in the prevalence of diabetes over the past several decades is apparently more likely to be due to insulin resistance (diminished insulin action on target tissues). The current epidemic of Type II diabetes in the United States is usually attributed to the aging of the population, the increased prevalence of obesity and sedentary activity, and the enrichment of the population with ethnic groups that may have a genetically predisposed inability of the pancreas to meet the challenge of increased insulin resistance or pancreatic dysfunction. The high incidence of diabetes represents a significant economic burden, such that approximately $92 billion in health care expenditures in 1992 were diverted to the treatment of diabetes.
Insulin resistance is a key factor in the pathogenesis of Type II diabetes, and can precede by decades abnormal insulin secretion and the onset of clinical diabetes. Resistance to insulin action involves all major target tissues, i.e., skeletal muscle, liver and fat. Although insulin resistance appears to involve defects in insulin signaling at the post-receptor level, the mechanism of insulin resistance remains poorly understood.
The action of insulin is initiated by binding to cell surface receptors. Autophosphorylation and activation of the intrinsic tyrosine kinase of the insulin receptor β-subunit leads to phosphorylation of several proximal interacting proteins, including insulin receptor substrate-1 (IRS-1), IRS-2, and Shc. IRS-1 interacts with several proteins that contain Src homology 2 (SH2) domains, including the p85 subunits of PI3′-kinase, GRB-2, Syp and Nck. Activation of these proteins and the subsequent cascade activation of other intracellular signaling molecules, such as p21ras, raf-1, MAP kinases, and S6 kinase, account for many of insulin's pleiotropic effects. Each of these cytoplasmic substrates and the activating regulatory loop involved represents a potential linkage to the development of insulin resistance.
The substantial number of signaling circuits involved, including interacting, bypassing and overlapping pathways, the involvement of numerous serine/threonine kinases and phosphatases, and still uncharacterized links, characterize the complexity of the signaling from the insulin signal at the cell surface receptor to targets within the cell. One approach to the study of insulin interactions with cells is to select a physiological action of insulin and then trace back toward the receptor, an approach known as the target backward approach. This target backward approach has yielded information concerning the mechanism of insulin regulation by focusing on the genetic regulation of the insulin-regulated gene insulin-like growth factor binding protein-3 (IGFBP-3).
Genetic factors also contribute to the development of non-insulin dependent Type II diabetes mellitus (NIDDM). The concordance rate for NIDDM in identical twins approaches 100%, while the risk to other siblings of a diabetic proband is between 30 and 40%. Despite considerable investigative efforts, the genetic heterogeneity of diabetes and the contribution of environmental factors in the development of the phenotype make the identification of specific diabetes-related genes difficult. Methods used in the study of the genetics of NIDDM include association of case control studies, positional searches, parametric linkage, and molecular screening using single-strand conformation polymorphism analysis. In addition, cloned genes, including genes important for both insulin secretion and insulin action, have been examined for sequence abnormalities. Specific mutations associated with insulin resistance and the development of diabetes have been identified for the α- and β-subunits of the insulin receptor. Rad (Ras-associated with diabetes), and the glucokinase gene implicated in MODY (maturity onset diabetes of the young), as well as HNF-1 and HNF-4. Such mutations, however, appear to account for less than 5% of patients with Type II diabetes.
A series of adapter proteins or substrates link the receptor tyrosine kinases to gene transcription, and determine the response to insulin in a given cell or tissue. Each of the proteins in the signaling cascade is a potential candidate for an acquired or genetic defect contributing to insulin resistance. Thus, characterization of the insulin-responsive binding proteins (IRBPs) that may bind to gene transcriptional regulatory sequences essential for insulin-regulated expression of target genes, and delineation of the pattern of signal transduction to the IRBPs constitutes an important strategy to identify genes important in mediating insulin resistance.
Insulin-like growth factors I and II (IGF-I and -II) are proteins that have insulin-like metabolic and trophic effects and mediate some of the peripheral actions of growth hormone. IGFs also have a role in wound healing by stimulating fibroblasts to produce collagen and induce hematopoiesis through an erythropoietin-like activity. Studies have also shown that certain cancer cells, such as from breast and kidney, produce IGFs. IGF production in cancer cells auto-regulates cell proliferation and the production of a vascular system required for growth of the tumor mass. IGFs have also been implicated in diabetic retinopathy by stimulating endothelial and fibroblast proliferation.
The actions of IGFs are modulated by a family of six IGF-binding proteins (IGFBPs) that have different tissue distribution and production sites. One binding protein, IGFBP-1, has a molecular weight of approximately 30-40 kd in the human and the rat. Most of the circulating plasma IGF-I and IGF-II, however, are associated with IGFBP-3 and an acid-labile subunit thereof that serve as reservoirs for IGFs. Diabetes mellitus in humans and animal models is associated with decreased levels of serum IGFBP-3. Hepatic expression of IGFBP-3 is correlated with circulating IGFBP-3 levels in streptozotocin-diabetic and BB/W rats. Thus, hepatic expression of IGFBP-3 appears to determine systemic IGFBP-3 levels; and the study of the mechanisms by which insulin stimulates hepatic synthesis of IGFBP-3 is critical for understanding the regulation of systemic IGFBP-3.
Most evidence indicates that IGFBP-3 is inhibitory to IGF action. Furthermore, IGFBP-3 can: (a) mediate the growth inhibitory actions of transforming growth factor-β (TGF-β), retinoic acid, anti-estrogens and fibroblast growth factor, (b) mediate the induction of apoptosis by the tumor suppressor gene p53, and (c) travel to the cell nucleus, potentially directly regulating the transcription of critical growth inhibitory genes independent of IGF-1.
The levels of IGFBP-3 in serum and liver mRNA are highest during puberty and adult life. Unlike other IGFBPs, IGFBP-3 levels increase in the presence of anabolic hormones such as insulin and growth hormone. Dependence on growth hormone (GH) has been inferred from the deceased levels of IGFBP-3 in hypopituitary subjects and GH-deficient children and increased levels in acromegalic patients. Additionally, IGFBP-3 production is inhibited at the level of gene expression by glucocorticoids.
The mechanisms by which IGFBP-3 is regulated are complex. IGFBP-3 may undergo post-translational processing to yield various proteolytically cleaved, phosphorylated, and glycosylated products. These processes have been shown to alter the binding of IGFBP-3 to the acid-labile subunit, cell surfaces and to affect the affinity of IGFBP-3 for IGFs. IGFBP-3 can also associate with the cell surface and extracellular matrix; dissociation of cell-associated IGFBP-3 is one mechanism by which IGF-1 promotes release of IGFBP-3 into conditioned medium by fibroblasts and breast cancer cells.
Insulin increases IGFBP-3 expression by stimulating the rate of gene transcription rather than by stabilization of mRNA transcripts. This enhancement is mediated through a cis-regulatory insulin-responsive element (IRE) localized to the −1150 to −1124 bp region of the gene encoding IGFBP-3. The IGFBP-3 IRE comprises the nucleotide dyad ACC(A/G)A which has a strong resemblance to the recognition sequence of ETS-related transcription factors, namely AGGAA, which is within the IRE of both the prolactin and somatostatin genes. The 10-bp core sequence of the IGFBP-3 IRE that is most critical for insulin responses (base positions −1148 to −1139) had no significant consensus sequence similarity to previously identified transcription factor binding sites. What was not known, however, was any protein or other factor that would mediate a cellular response to insulin and which directly binds to such insulin-response elements like the IRE of IGFBP-3.